Hi there,

This is a frequent issue because not all the original 3DS files are developed in the same way. Some of them are created with bad programs, therefore, the quality is low.
I created a tutorial to import 3DS objects: you will find it in my website
http://www.punchhelpers.com/3dsfiles.html
or in my FREE eBook
http://www.smashwords.com/books/view/369372

If you are using Accutrans 3D, merge the layers (in the Tools menu) and reduce the polygons.

Patricia
--> My website: www.punchhelpers.com
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